More and more motor vehicles now have, for safety purposes, monitoring systems that include sensors fitted on each of the wheels of the vehicle, dedicated to measuring parameters, such as pressure or temperature of the tyres of the wheels, and intended to inform the driver of any abnormal variation of the measured parameters.
These monitoring systems usually include a sensor placed in each wheel with a microprocessor and a radiofrequency transmitter, as well as a central processing unit for receiving signals sent by the transmitters, in which the central processing unit includes a computer incorporating a radiofrequency receiver connected to an antenna.
One of the problems that such monitoring systems have to resolve lies in the obligation of having to associate, with each signal received by the receiver of the central processing unit, an indication concerning the location of the sensor and therefore of the wheel from where the signal originates, this obligation lasting throughout the life of the vehicle, even after wheel changes or simple reversals of the positions of the wheels.
The document EP 1 669 221 A1 presents a stand-alone device intended to be supported by a wheel for locating the right or left position of the wheel in a vehicle. The device includes:                two magnetic sensors with maximum sensitivity axes;        means of measuring a signal at the terminals of each of the magnetic sensors, the means being able to deliver two periodic signals phase-shifted relative to each other and each representative of magnetic flux time variations detected by the magnetic sensors during rotation of the wheel; and        a computation unit programmed to determine, from the phase shift between the two periodic signals, a direction of rotation of the wheel.        
The knowledge of the direction of rotation of the wheel makes it possible to deduce, given that a direction of movement of the vehicle is known otherwise, the right or left location of the wheel.
The above device is configured such that the maximum sensitivity axes of he two magnetic sensors are intended to be placed in the tyre and wheel assembly in a secant plane relative to the axis of rotation of the wheel (i.e., an assembly of the wheel and a tyre), and offset relative to each other, in this secant plane, by a predetermined angle.
The two sensors of this device detect a global magnetic field surrounding them, consisting of the earth's magnetic field added to the vehicle's environmental magnetic field. The vehicle's environmental magnetic field is the resultant of a set of vehicle fields created by the presence of electrical or magnetic equipment installed in the vehicle and by metal parts close to wheel housings, such as brake callipers and elements linking to the ground.
There are, however, points on the terrestrial globe and directions of movement of the vehicle for which the contribution of the earth's magnetic field measured by the two sensors of the device become negligible relative to the contribution of the environmental magnetic field.
Devices such as that described above make it possible to locate only the left or right position of a wheel on a vehicle, but not the front or rear position.
Patent application publication WO 2006/104484 discloses a device and a method for identifying the position of a wheel of a vehicle, in which each wheel housing is fitted with specific magnetic field generators and each wheel is fitted with a magnetic field sensor. When the vehicle moves, each magnetic field sensor delivers a signal linked to the magnetic field generators specific to the position of the wheel, and analysis of this signal makes it possible to identify the front or rear and left or right position of this wheel.